Brushes, Hammers, Paste and Nails
I have a colleague who uses this "viewing frame" type of thing. He uses it often in relation to creating his work. His is just a 'window mount' cut from mount board (as would be used for placing over a work on paper, before putting it into a glass frame). I would just make one as opposed to buying one......although your local framing shop may have some 'seconds' with stains or slight defects, that they might let you have for free or for a very low cost.